(1) Field of the Invention
The present invention generally relates to underfill processes and materials for flip-chip mounted dies. More particularly, this invention relates to a process for selectively depositing a filled, wafer-applied underfill material on a die prior to die attachment and without covering solder bumps present on the die.
(2) Description of the Related Art
Underfilling is well known for promoting the reliability of surface-mount components, such as flip chips (chip scale packages, or CSP's) and ball grid array (BGA) packages, that are physically and electrically connected to traces on organic or inorganic substrates with numerous solder bump connections. Conventional underfill processes generally involve using a specially formulated dielectric material to completely fill the void between the component and the substrate and encapsulate the solder bump connections of the component. In conventional practice, underfilling takes place after the component is attached to the substrate. The underfill material is placed along the perimeter of the component, and capillary action is relied on to draw the material beneath the component. An alternative technique is to deposit a no-flow underfill material on the substrate surface, place the bumped die on the substrate (forcing the solder bumps through the underfill material), and then attach the die by reflow soldering. Another underfill method is wafer-applied underfilling (WAU), in which a film of underfill material is laminated to a wafer or chip prior to solder bumping or to a bumped wafer or chip prior to die attachment. Yet another technique disclosed in U.S. Pat. No. 5,681,757 is to deposit an underfill material on the die prior to die attachment through the use of a microjetting (inkjet) process, in which droplets of the underfill material are deposited on the die surface between solder bumps. This technique is represented in FIGS. 1 and 2, which shows a die 110 with solder bumps 112 and an underfill material 114 on the die surface between bumps 112. The die 110 is registered with bond pads 118 on a substrate 116, and the solder bumps 112 reflowed to form solder connections 120.
For optimum reliability, the composition of an underfill material and the underfill process parameters must be carefully controlled so that voids will not occur in the underfill material beneath the component, and to ensure that a uniform fillet is formed along the entire perimeter of the component. Capillary-flow underfilling processes can generally be performed to ensure an adequate and uniform fillet, which has been shown to be an essential factor in terms of the thermal cycle fatigue resistance of the solder connections encapsulated by the underfill. However, obtaining a void-free underfill using a capillary-flow technique can be difficult if the die has a low standoff height and/or has closely-spaced solder bumps. In contrast, the microjetting process of U.S. Pat. No. 5,681,757 generally ensures a void-free underfill, but the amount of underfill material that can be applied without covering the solder bumps is insufficient to form an adequate and uniform fillet. Such an inadequate fillet 128 is represented in FIG. 2, in that the fillet 128 does not extend up along the peripheral wall 130 of the die 110.
In addition to the above considerations, underfill materials must have a coefficient of thermal expansion (CTE) that is relatively close to that of the solder connections, component and substrate. As known in the art, an acceptable CTE match is necessary to minimize CTE mismatches that reduce the thermal fatigue life of the solder connections. Dielectric materials having suitable flow and processing characteristics for underfill applications are typically thermosetting polymers such as epoxies. To achieve an acceptable CTE, a fine particulate filler material such as silica is added to the underfill material to lower the CTE from that of the polymer to something that is more compatible with the CTE's of the component, substrate, and-the solder composition of the solder connections. Suitable fill levels and compositions for the filler material are dependent on the particular polymer used and the amount and size of filler material necessary to achieve the desired CTE.
While highly-filled capillary-flow underfill materials have been widely and successfully used in flip chip assembly processes, expensive process steps are typically required to repeatably produce void-free underfills. These steps can limit the versatility of the flip chip underfill process to the extent that capillary-flow underfilling is not practical for many flip chip applications, especially those chips with fine pitch solder connections and low standoff heights. These applications are candidates for no-flow and wafer-applied underfill materials. However, a drawback of wafer-applied underfilling techniques is that, depending on when the underfill material is applied, the bond pads or the solder bumps present on the wafer or chip must be re-exposed prior to die attachment, such as by burnishing or a laser ablation process. Furthermore, a drawback of no-flow underfill materials is that they typically do not contain filler materials because of the tendency for the filler particles to hinder the flip chip assembly process. For example, particles can become trapped between the solder bumps and the bond pads to interfere with the formation of a metallurgical bond, reducing the reliability of the electrical connection. With regard to U.S. Pat. No. 5,681,757, no mention is made of whether a filled underfill material is compatible with the disclosed microjetting process. However, conventional filler levels and filler materials capable of achieving an acceptable CTE for an underfill material are generally incompatible with inkjet technology because of the excessively high viscosity of such materials.
In view of the above, it would be desirable if an underfill process were available that overcame the difficulties and shortcomings of capillary-flow, no-flow and wafer-applied underfill materials and processes.